The present invention relates to a polyester composition which is capable of providing a film having excellent wear resistance and flatness. More particularly, the present invention relates to a polyester composition which permits lengthy film production runs and produces films having excellent wear resistance and flatness, which characteristics are in increasing demand for base films of magnetic recording media.
Polyester films, which are excellent in physical and chemical properties, have been widely used as a base material of various products. Above all, a biaxially oriented polyethylene terephthalate film, which is excellent especially in mechanical strength and dimensional stability, has now become essential as a base film of magnetic recording media. With the increase in the running speed of a magnetic recording tape, films must exhibit enhanced wear resistance. This is because scratches are readily produced on films which come into contact with a guide pin and the like at a high speed, and such scratches cause defects on the surface of a magnetic layer, thereby deteriorating the electromagnetic characteristics. In addition, a substance in the form of a white powder which is scraped off the film and which adheres to the surface of a magnetic layer also often deteriorates the magnetoelectric characteristics of the recording media.
As a result of investigation on the improvement of wear resistance, the present inventors have found that the presence of a specific amount of aluminum oxide particles having a specific particle diameter in a polyesters film is very effective for improving the wear resistance thereof. However, if a polyester composition containing such particles is used for film formation for long hours, a deteriorated substance is gradually produced on the lip portion of the die of an extruder, which produces stripes on the film, thereby impairing the flatness of the film.
As a result of research for solving this defect, the present inventors have found that the use of a polyester composition which comprises a specific amount of aluminum oxide particles and a polyester with a terminal carboxyl group content higher than that of an ordinary polyester and which has a melt resistivity of a specific range can stably produce a film having excellent wear resistance and flatness extending over long hours. The present invention has been achieved on the bases of this finding.